Optoelectronic devices typically include a semiconductor structure arranged for generating light when, a current is introduced through the structure. Optoelectronic devices typically have semiconductor structures incorporating thin layers of semiconductor materials exhibiting opposite conductivity types, referred to as p-type conductivity and n-type conductivity. The p-type and n-type semiconductor layers are typically disposed in a stack, one above the other so as, to form a junction with one another. In certain structures, the p-type layer is disposed at the top of the stack of semiconductor layers and the n-type layer is disposed at the bottom of the stack. As used herein, references to the top or bottom of any feature are to be taken with reference to the device itself, as opposed to any gravitational frame of reference, as the devices disclosed herein could be oriented in any direction. The junction between the p-type and n-type material may include directly abutting p-type and n-type layers, or may include one or more intermediate layers which may be of any conductivity type or which may have no distinct conductivity type. The junction may also include other structures.
Light is generated at the junction when an electric current is introduced to the structure. As used in this disclosure, the term “light” includes radiation in the infrared and ultraviolet wavelength range, as well as the visible range. The wavelength of the light depends on factors including the composition of the semiconductor materials and the structure of the junction. One electrode is mounted on the p-type layer and another electrode is mounted near or on the n-type layer so as to introduce a current through those layers and through the junction. The materials in the electrode are selected to form low-resistance interfaces with the semiconductor materials. The electrodes may include pads for forming connections with other conductors for carrying current from external sources. The term “electrode-pad unit” as used in this disclosure refers to the electrode and the pad, whether the pad is a separate structure or formed as part of the electrode, or comprises a region of the electrode.
Certain structures incorporate current-spreading electrodes. Current-spreading electrodes make broad contact with the semiconductor structure and cover a significant portion of the top surface of the stack. Such electrodes, spread the current delivered to the top surface of the stack so that the effects of so-called. “current bunching” or “current crowding” are reduced. Current bunching is the tendency for current to travel straight through the semiconductor structure in a downward direction. Current is concentrated in that portion of the junction beneath the electrode. The light is generated in only the region accessed by the current. Where the electrode covers only a relatively small portion of the top surface of the stack, the amount of useful light reaching the outside of the optoelectronic structure per unit of electrical current passing through the structure, commonly referred to the external quantum efficiency of the structure, is reduced by current bunching. Thus, it is desirable to spread the current introduced to the semiconductor structure. In certain devices, a current-spreading electrode is often placed on a major surface of the device. Conductive materials utilized for electrodes in many contexts typically comprise metals that are opaque to light. Since a current-spreading electrode covers a major surface of the structure, the light generated by the structure does not pass through this surface if an opaque metal electrode is used. In certain applications, it is desirable to utilize a transparent electrode, so that the light can pass through the transparent electrode.
Certain optoelectronic devices, such as light-emitting diodes (LEDs), are formed with a region of the semiconductor structure removed so that an upwardly facing lower region is formed and an upwardly protruding region, referred to as a mesa, is formed. The top of the mesa typically comprises the top of a p-type semiconductor material on which a top electrode is disposed. A lower electrode is formed on the lower region, near the n-type layer. This structure is typically formed on a substrate or is otherwise mounted to a substrate or other support beneath the n-type layer so that light is directed out the top of the structure. Reflectors may be included in the support or within the semiconductor structure itself to direct light upwardly. In these structures, it is desirable that the top electrode is comprised of a substantially transparent material so that at least a portion of the light emitted by the LED shines out the top of the structure, in addition to the sides. It is preferred that the top electrode is as transparent to the light generated as possible.
In forming an optoelectronic device having a mesa, the mesa is typically formed first and then the p-type electrode is formed on the mesa. A first resist is applied to the structure and photolithographically patterned to form openings over certain regions of the structure. After patterning, the first resist remains on those regions of the wafer corresponding to the areas where mesas are to be formed. The regions of the semiconductor structure that are aligned with the openings in the first resist are removed by etching. The regions covered by the resist remain as mesas, which protrude from the structure. The first resist is stripped and a second resist is applied to the structure, and patterned to form openings for the p-type electrodes on the top of the mesas. Metal is then deposited in these openings. The metal deposited on the top of the mesa may be comprised of one or more metals that are selected to form a transparent electrode, upon subsequent annealing of the metals.
The inventors have, found that the foregoing process interferes with the optimum development of a transparent electrode. Without committing to any theory of operation, it is believed that the use of the resist layers on the top of the semiconductor structure interferes with achieving transparency after annealing. In addition, the second resist must be carefully registered with the mesas to avoid malformed p-type electrodes. If an opening in the second resist extends over an edge of a mesa due to misregistration, the resulting top electrode will contact the lower electrode on the lower region, thus shorting out the device. It is preferred that the top electrode is as transparent to the light generated as possible. It is also desirable to form the top electrode so that the edges of the electrode are spaced from the edges of the mesa, also to avoid shorting out the device.
Improvements in the methods of forming electrodes on semiconductor structures to address the foregoing issues are desirable.